Abstract

The Park Grass Experiment (PGE) is the longest observed set of experimental plant communities in existence. Although the gross composition of the vegetation was at equilibrium over the 60-yr period from 1920 to 1979, annual records show that individual species exhibited a range of dynamics. We tested two hypotheses to explain why some species initially increased nd why subsequently some of these (the outbreak species) decreased gain. The study was designed around eight phylogenetically ndependent contrasts (PICs), each containing related species with ifferent dynamics. Our first hypothesis was that persistent increasers and utbreakers have higher intrinsic rates of natural increase than ontrol species (species without trends), allowing them to spread hen interspecific competition is reduced by drought. This was tested by measuring establishment and seed production of species in ield experiments, with and without interspecific competition. Seed production in outbreak species responded more strongly to release from interspecific competition than it did in either of the ther groups of species. Our second hypothesis was that outbreak species eventually declined because they lacked the genetic variation ecessary to adapt to the novel habitats to which they had initially spread. We tested this by measuring mating systems and genetic diversity in persistent and outbreak species in the PGE. In seven out of seven PICs tested, the outbreak species was more selfing than its persistent relative. There was a significant positive correlation between outcrossing rate and gene diversity. These results support roles for both ecological and genetic traits in long-term dynamics.